Conventionally, when image data is transmitted from one network or system to another, the image data is typically transmitted as bi-tonal image data. Bi-tonal transmission enables a fast and efficient way for transmitting image data between systems. An example of such a transfer of image data between systems is the typical telefax transmission which transmits image data in a bi-tonal manner.
However, the system receiving the image data usually processes image data having a bit resolution which is greater than that which is transmitted; i.e., eight bits per pixel. Thus, to facilitate the processing of the transmitted image, it is necessary to expand the grey value of the bi-tonal image to the native bit resolution of the receiving system. In other words, the bi-tonal image data must be expanded to the bit resolution of a receiving system in order to take full advantage of conventional image processing functions in the receiving system; i.e. image enhancement, contrast adjustment, etc.
Moreover, grey value expansion or conversion is also required for transmitted multi-bit resolution pixel image bytes when the number of bits per pixel byte (bit resolution) of the received image data is less than the native bit resolution of the receiving system.
Resolution of an image is generally set forth as A.times.B.times.C wherein A is the number of pixels per inch in a fast scan or electronic scanning direction; B is the number of pixels per inch in the slow scan or mechanical scanning direction; and C is the number of bits per pixel (grey value). In the present application, the term "bit resolution" is utilized to describe the resolution parameter C, the number of bits per pixel. Thus, as defined in the present specification, expanding the grey value of a pixel is the increasing of the bit resolution of the image data by increasing the number of bits defining a pixel.
Conventionally, grey value expansion has been done for bi-tonal image data by directly mapping a bi-tonal zero to all zeros and a bi-tonal one to all ones. For example, as illustrated in Table 1 below, if the bi-tonal pixel value is a logic zero and this byte of image data is to be converted to eight bits, the output of the conventional mapping conversion would be an eight bit pixel byte having all logic zeros. Moreover, if the pixel value was a logic one and this byte of image data is to be converted to eight bits, the conventional direct mapping method would output an eight bit pixel byte containing all logic ones.
TABLE 1 ______________________________________ NEW PIXEL NEW PIXEL VALUE BY VALUE BY ORIGINAL DIRECT ORIGINAL DIRECT PIXEL VALUE MAPPING PIXEL VALUE MAPPING ______________________________________ 0 00000000 1 11111111 ______________________________________
If the received image data to be converted/expanded has a bit resolution corresponding to a multi-bit pixel value; i.e. two bits or four bits per pixel byte; conventionally, the multi-bit pixel value is expanded to an eight bit pixel byte grey value by a padding method. More specifically, for the received multi-bit pixel value, the original pixel bits are placed as the most significant bits of the new pixel byte and the least significant bits are padded with zeros. Table 2 below clearly illustrates the conventional padding conversion method wherein the originally received pixel byte is converted to the new pixel byte by zero padding. For example, as shown in Table 2 below, if the original received pixel byte is 10, the new pixel byte utilizing the padding method would be 10000000. Moreover, if the originally received pixel byte is 0111, the new pixel byte from the padding method would be 01110000. This conventional padding method can be realized by a simple multiplication operation (the pixel byte multiplied by 16 or 10000) or load/shift/load register operation.
TABLE 2 ______________________________________ ORIGINAL NEW PIXEL ORIGINAL NEW PIXEL PIXEL BYTE BYTE PIXEL BYTE BYTE ______________________________________ 00 00000000 0110 01100000 01 01000000 0111 01110000 10 10000000 1000 10000000 11 11000000 1001 10010000 0000 00000000 1010 10100000 0001 00010000 1011 10110000 0010 00100000 1100 11000000 0011 00110000 1101 11010000 0100 01000000 1110 11100000 0101 01010000 1111 11110000 ______________________________________
A problem with the conventional methods of expanding the grey value bit resolution of a pixel is that the direct mapping method and the padding method result in a reduced dynamic range for the new pixel byte. Subsequently, any image processing operation performed on the new pixel byte will not be optimized. Therefore, the present invention proposes a grey value expansion method or system which easily converts the lower bit resolution pixel byte into a new higher bit resolution pixel byte without significantly reducing the dynamic range of the converted pixel image data.